英语论文网

ng RJ - 45 standard, and the connection to use ISDNNT1 RJ - 45 - to - RJ - 45 straight-through. As shown in figure for ISDN BRI port.
3.3.4 WAN connection type:
Leased Lines: general Leased is special point-to-point circuit, for flow relatively fixed and closer distances occasions, cost is higher, (currently can to China telecom, China mobile, China unicom, radio and television companies such as leased lines).
Circuit switching: according to the demand Switched connection is established and allow for these connections until they released such a process, such as telephone network.
Packet switching/Packet switching: sharing lines. Use of packet-switched way connectingwan, need to build a virtual circuit.
Broadband Access (Broadband Access) : ADSL and Cable.
3.4 The IEEE MAC Sub-Layer
The IEEE MAC layer contains protocols that control access to a shared medium. Channelization protocols consist of extensions to time, frequency,and code division multiplexing; the extensions are known as Frequency,Time, and Code Division Multi-Access.Static or dynamic channel allocation is possible.
Controlled access protocols allow independent stations to engage in statistical multiplexing. Polling uses a central controller that repeatedly checks whether stations are ready to send a paket. A reservation system,often used with satellites, requires stations to declare whether they are ready for the next round of transmission. Token passing, often used with a ring topology,passes a control message among stations; a station can transmit a packet when it receives the token.
Random access protocols allow stations to contend for access. The historic ALOHA protocol used two frequencies, one for inbound and one for outbound transmissions; if a station did not receive a copy of its packet ,the station retransmitted.Ethernet uses Carrier Sense Multi-Access with Collision Detection(CSMA/CD)to permit access to a shared cable. In addition to preventing a station from transmitting while another transmission is in progress, the protocol uses binary exponential backoff to recover from collisions.
Because some stations are hidden from others, wireless LANs use Carrier Sense Multi-Access with Collision Avoidance(CAMA/CA). Before transmission of a packet from one computer to another, each of the two computers sends a short control message, which allows all computers in range of the two to know that a transmission is about to occur.
3.4.1 CSMA/CD
In 1973, researchers at Xerox PARC created an extremely successful network technology that used a random access protocol. In 1978，a standard (informally called the DIX standard) was created by Digital Equipment Corporation, Intel, and Xerox. Known as Ethernet, the original Ethernet technology consisted of a single long canle to which computers attach. The cable served as a shared medium---instead of broadcasting radio frequency transmissions through the atmosphere, Ethernet transmitted signals down a cable. Furthermore, instead of using two freuencies and a central transmitter, Ethernet allows all communication to proceed across the shared cable.Despite their differences,Ethernet and ALONAnet had to solve the same basic problem:if two stations attempt to transmit at the same time, the signals interfere and a collision occurs.
Ethernet offered three innovations in the way collisions are handled:
•Carrier sense
•Colli